Text 9 the properties of building materials

Materials that are used for structural purposes should meet several requirements. In most cases it is important that they should be hard, durable, fire-resistant and easily fastened together.

The most commonly used materials are steel, concrete, stone, wood and brick. They differ in hardness, durability and fire-resistance.

Wood is the most ancient structural material. It is light, cheap and easy work. But wood has certain disadvantages: it burns and decays.

Stone belongs to one of the oldest building materials used by man. It is characteristic of many properties. They are mechanical strength, compactness, porosity, sound and heat insulation, and fire-resistance.

Bricks were known many thousands of years ago. They are the exam­ples of artificial building materials.

Concrete is referred to as one of the most important building mate­rials. Concrete is a mixture of cement, sand, crushed stone and water.

Steel has come into general use with the development of industry. Its manufacture requires special equipment and skilled labour.

Plastics combine all the fine characteristics of a building material with good insulating properties. If is no wonder that the architects and engineers have turned to them to add beauty to modern homes and offices.

All building materials are divided into three main groups: 1) Main building materials such as rocks arid artificial stones, timber and materials. 2) Binding materials such as lime, gypsum and cement. 3) Secondary of auxiliary materials which are used for the interior parts of the buildings.

We use many building materials for bearing structures. Binding materials are used for making artificial stone and for joining different planes. For the interior finish of the building we use secondary materials.

Natural building materials are: stone, sand, lime and timber. Cement, clay products and concrete are examples of artificial building materials.

Text 10 reinforced concrete

Reinforced concrete is a combination of two of the strongest struc­tural materials, concrete and steel.

This term is applied to .a construction in which steel bars or heavy steel mesh are properly embeded in concrete. The steel is put in position and concrete is poured around and over it, then tamped in place so that the steel is completely embeded. When the concrete hardens and sets, the resulting material gains great strength» This new structural concrete came into practical application at the turn of the 19th century. The first results of the tests of the reinforced concrete beams were pub­lished in 1887. Since that time the development of reinforced concrete work has made great progress. And the reasons of this progress are quite evident. Concrete has poor elastic and tensional properties, but it is rigid, strong in compression, durable under and above ground and in the presence or absence of air and water, it increases its strength with age, it is fireproof.

Steel has great tensional, compressive and elastic properties, but it is not durable being exposed to moisture, it loses its strength with age, or being subjected to high temperature. So, what is the effect of the ad­dition of steel reinforcement to concrete?

Steel does not undergo shrinkage or drying but concrete does and therefore the steel acts as a restraining medium in a reinforced concrete member. Shrinkage causes tensile stresses in the concrete which are balanced by compressive stresses in the steel. For getting the best from reinforced concrete the following consideration should be kept in mind:

1. For general use the most suitable proportions of cement and ag­gregate are: 1 part cement, 2 parts sand and 4 parts of gravel.

2. Only fresh water free from organic matter should be used for rein­forced work. Sea water is not allowed.

3. Homogeneity of the concrete is a very important requirement. Steel constructions with reinforced concrete have become the most important building materials invented in centuries and they have given modern architecture its peculiar features.